Zinc Aryl/Alkyl β-diketiminates: Balancing Accessibility and Stability for High-Activity Ring-Opening Polymerization ofrac-Lactide

The development of nontoxic initiators which display both high activity and stability at low catalyst concentrations can be challenging, as these desired attributes often involve antagonistic design elements. In this study, we report a family of N-aryl/N′-alkyl zinc β-diketiminate catalysts, Zn(Ar/AlkL)[N(SiMe3)2] and [Zn(Dipp/BnL)(OiPr)]2, which can overcome historic challenges encountered with traditional zinc systems in the ring-opening polymerization (ROP) of rac-lactide and l-lactide. The disymmetric 2,6-diisopropylphenyl (Dipp)/benzyl (Bn) derivative, Zn(Dipp/BnL)[N(SiMe3)2], can access polylactic acid (PLA) with a wide range of molecular weights in the presence of a chain transfer agent (e.g., isopropanol; Mn(PLA) = 7–521 kg/mol, D̵ = 1.03–1.07). ROP proceeds rapidly at RT with high turnover frequencies (TOFs = 13,950 h–1 at 0.1 mol % [Zn]; [lactide] = 1.0 M) and low catalyst loadings (down to 0.02 mol %) and enables the synthesis of narrow dispersity ultrahigh molecular weight PLA (UHMW-PLA). Our detailed mechanistic studies identify a unifying and quantitative structure–function relationship for a diverse range of Zn(R1/R2L)[N(SiMe3)2]. This takes the form of a volcano plot, which links the catalyst’s reaction site steric profile (Vb,RS) with catalyst activity in the ROP of rac-lactide [log(kp)]. Our results establish the utility of disymmetric designs in accessing catalysts with high activity and stability at low concentrations and the prevalence of Sabatier’s principle and volcano plots in the design of homogeneous catalysts for ROP.